I've tried using the threaded inserts designed for wood but with poor results in fiberglass. Those are the ones that are a SS or brass barrel with machine screw threads on the inside and some wood-screw type threads on the outside. When I mounted hinges for a new companionway to the fiberglass sole I wanted good strength. I drilled and tapped some 3/8" brass pipe plugs with 1/4" threads after cutting off the square head. This made sort of an insert on steroids. Gooped them with 5200 and set them in using the 1/4" bolt as a driver. So far they are holding well. If you have saltwater exposure you might want to go with bronze or SS.